The role of enterotoxin-producting E. coli (EEC) in the etiology of diarrheal illness will be explored using the highly sensitive Chinese hamster ovary (CHO) cell assay of heat labile enterotoxin (LT) and the infant mouse assay of heat stable enterotoxin (ST). The CHO cell assay developed by the P^I^ is efficient in screening many coliform isolates for LT production. We shall examine attack rates of gastrointestinal illness, prevalence of EEC, serotype distribution and antitoxic immunity in a family population under close surveillance, and study association of EEC with diarrheal illness in varied patient populations and matched controls. Search for other enterotoxigenic coliform bacteria, "traditional" enteric pathogens and clinical and laboratory correlates will be made.